1. Field of the Invention
The present invention relates to a manufacturing method for a semiconductor device and, more particularly, to a manufacturing method for a semiconductor device which has multilayer interconnection.
2. Description of the Related Art
As an interlayer insulating film of a semiconductor device, a polyimide-based resin layer such as a polyimide layer or a polyimide layer to which silicon has been added (hereinafter referred to as "silicon polyimide layer") is used primarily because of the good flatness, low dielectric constant, and simple manufacturing process thereof. The polyimide-based layer, however, is disadvantageous in that moisture or chemicals used during the manufacturing process of semiconductor devices soak therein, requiring such moisture, etc. be diffused outside from the polyimide-based layer and degassing be carried out in an appropriate process. A failure to perform the degassing treatment results in the occurrence of blisters which lead to such damage as a cracked passivation and bulging wires due to the heat generated during the heat treatment after depositing a passivation film or the heat generated in operation.
A prior art for the foregoing process will be explained by reference to FIGS. 1A-1D. As shown in FIG. 1A, first-layer aluminum type wires 2-1 and 2-2 made of, for example, Al--Si--Cu alloy films are formed on a first insulating film 1 composed of a BPSG film or the like on a semiconductor substrate; an oxide silicon film or a silicon nitride film which is approximately 100 nm thick is deposited thereon by plasma chemical vapor deposition (CVD) to form a second insulating film 3; and a liquid material (e.g. LIXON coat, PIN-6001 made by Chisso K.K.) is applied thereto and subjected to heat curing to form, for example, a silicon polyimide layer 4. Next, as illustrated in FIG. 1B, a through hole 5 is formed and a second-layer aluminum type wire 6 is formed in the through hole 5 as illustrated in FIG. 1C. Then, as illustrated in FIG. 1D, a silicon nitride film 7 is deposited to a thickness of about 500 to about 1000 nm by the plasma CVD so as to produce the cover film. Subsequently, the cover film on a bonding pad, which is not shown, is removed. The semiconductor device completed after pelletizing and mounting has been presenting a problem in that it develops a blister 10 or cracks 11 as shown in FIG. 2. These troubles are considered due to the chemicals or moisture adhering to or taken into the silicon polyimide layer 4 when the through hole 5 or the second-layer aluminum type wire 6 is formed; hence, it is necessary to perform heat treatment at 400 to 450 degrees Celsius in, for example, a nitrogen atmosphere immediately before forming the silicon nitride film 7.
An example of such heat treatment has been disclosed in Japanese Unexamined Patent Publication No. 3-131028: it was found effective to form the silicon nitride film 7 without exposing it to open air after heating while pressure was being reduced, whereas, if the silicon nitride film 7 was formed after exposing it to open air following the heating, then no remarkable effect was observed although the troubles described above were decreased about 20% to about 30%.
According to the art disclosed in the foregoing Japanese Unexamined Patent Publication No. 3-131028, the cover film must be formed without exposure to open air during pressure reduction after heating. This presents a problem in that the efficiency for using CVD is low, leading to a disadvantage in process control. Further, in this art, the heating and degassing are performed with the polyimide layer exposed; therefore, reaction products or the like resulting from the progress of the amidation of the silicon polyimide layer adhere to a furnace core pipe or other pipes. This poses a problem in that the parts involved must be cleaned or replaced, frequently leading to lower productivity, or lower yield or deteriorated reliability due to the occurrence of refuse.
Japanese Unexamined Patent Publication No. 3-125461 has disclosed a technique in which heat treatment is performed in an atmosphere where O.sub.3 is added to N.sub.2 to promote the reaction of an undecomposed gas in a SOG film. This technique could be applied to the degassing process of the polyimide-based resin layer; however, no satisfactory effect can be expected because the exposure to atmospheric air after degassing undesirably allows the moisture contained in the atmospheric air to get into the polyimide-based resin layer again.